In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member. The latent image is then rendered visible by the application of electroscopic marking particles, commonly referred to as toner, to the photosensitive member. The visual image is then transferred from the photosensitive member to a sheet of plain paper with subsequent permanent bonding of the image thereto. This bonding of the toner particles onto the paper generally comprises two steps: fusing, in which the toner particles on the paper are partially melted, or otherwise made fluid; and fixing, in which the fluid toner particles are bonded to the paper. In common parlance, however, these two steps are conceptually combined (since, in many techniques, the two steps occur substantially simultaneously), and the two steps are together known in the art simply as "fusing."
In order to fuse the image formed by the toner onto the paper, electrophotographic printers incorporate a device commonly called a fuser. While the fuser may take many forms, heat or combination hear-pressure fusers are currently most common. One combination heat-pressure fuser includes a heat fusing roll in physical contact with a relatively soft pressure roll. These rolls cooperate to form a fusing nip through which the copy sheet (the sheet on which the document is finally formed) passes.
Fuser rolls are typically in the form of a rotating cylinder, with an outer surface comprising a thin elastomeric layer which contacts the copy material. The outer surface may include a release material, such as the synthetic polymer resin known under the trade name "Teflon," to prevent toner from adhering to the surface of the fuser roll itself. Fuser rolls in common use have outer layers of a thickness on the order of 0.005-0.01 inches, while typical pressures exerted on the outer layer of a fuser roll are on the order of 50 to 150 psi.
It has been found that over an extended operating period, the copy material itself can cause excessive wear on certain portions of the fuser roll surface, most notably along the line where the relatively sharp edges of the copy material contact the fuser roll. The pressures associated with the fusing process create a stress line on the elastomeric layer along the edges of a sheet of copy material passing through the nip. When such stresses are repeated over thousands of sheets, a concentrated area of intense wear will result at each of the two points on the fuser roll corresponding to the edges of the sheets passing through. This problem is perhaps furthered by the tendency in the industry toward common sheet sizes, such as 11 inches. It is common among electrophotographic printers to feed 11-inch wide sheets through 14-inch wide rolls, because many designs preserve the option of feeding legal size (8.5" by 14") sheets through the fusing station in a long-edge feed manner. These areas of concentrated wear will clearly have a detrimental effect on the overall durability of the fuser roll.
U.S. Pat. No. 3,856,461 to Jordan, assigned to the assignee of the present application, discloses one proposed method for obviating the problems of wear on the fuser rolls. In this invention, one fuser roll is supported for limited axial displacement relative to the other roll. This axial movement of one fuser roll relative to the other serves to offset spot wear on the surface of the fuser rolls by spreading out the area along the axis of the fuser roll which comes in contact with an edge of a sheet of copy material passing through the nip. This invention, however, requires a relatively sophisticated movable roll bearing structure, which includes a bearing lock to retain the bearing structure and one of the fuser rolls in a selected axial position.
It as an object of the present invention to provide a method of feeding sheets of copy material through a nip formed by fuser rolls, which tends to reduce the wear on the fuser roll that is concentrated in discrete areas of the fuser roll.
It is another object of the present invention to provide such a method which does not necessarily require the addition of extensive ancillary equipment to an electrophotographic printing apparatus.
Other objects will appear hereinafter.